The present invention relates to a tray comprising a base in which a plurality of cells are formed and a removable cover or lid for the tray. The cells in the tray may be used to contain various media for testing, experimental, and other purposes. Trays of this type may be used, for example, as culture trays for tissue cultures.
In trays of this type there is normally a clearance between the base and cover to allow air or other gas, e.g., CO2, to circulate to the cells. The clearance is normally provided by raised beads or ribs in the cover or base which hold the two parts in a spaced relationship. This type of tray is shown, for example, in U.S. Pat. Nos. 3,597,326 of Liner and 4,012,288 of Lyman et al. The cover or lid in U.S. Pat. No. 4,012,288 is held spaced from the base by a series of support posts on the base on which it rests. The lid itself has protruding ridges in its inner surface which extend around the edges of the individual cells when the cover is in place to help prevent contamination between the cells as a result of condensation. One problem with such trays is the so-called `edge effect`. The gas flow pattern is such that the outer and corner cells are exposed to a greater gas flow than the inner cells. When such trays are used as tissue culture trays, the outer trays will evaporate more quickly than that in the inner cells. This gives rise to significant evaporation losses, uneven culture results, and experimental errors. It has been shown that the edge effect is more enhanced at the corner cells than the side edge cells, and can produce evaporation losses of the order of 15% at the side edges and more than 25% at the corner cells. In an effort to avoid such errors some experimentalists in the past have simply left the outermost cells of such trays empty.